Wireless communication network technology continues to expand at a rapid pace, enabling increasingly sophisticated services via dramatic improvements in bandwidth, signal quality, geographic coverage, and the like. For example, multiple-cell High-Speed Downlink Packet Access (HSDPA) operation is being standardized by the 3rd Generation Partnership Program (3GPP). In particular, 3GPP Work Item Description RP-080490, “Dual-Cell HSDPA operation on adjacent carriers” specifies dual-cell operation using two adjacent carriers in the downlink (DL). In dual-cell HSDPA, separate data streams are modulated onto separate carriers, and transmitted together to User Equipment (UE) in the cell (dual-cell is thus synonymous with dual-carrier). The dual-cell HSDPA operation can improve the bit rate in terms of peak rate and average bit rate from the end-user's perspective. Dual-cell HSDPA has been included in 3GPP Release 8 specification, and some related extensions have been proposed for inclusion in Release 9. One such extension, aims at introducing dual-carrier operation also in the uplink (UL) direction, as described in the 3GPP Work Item Description RP-090014, “Dual-Cell HSUPA”.
HSDPA in legacy wireless networks is based on single cell, or carrier, operation. The DL frequency error between a transmitter and receiver—caused by frequency error in the base station and Doppler shift in the channel due to UE mobility—is compensated in the UE for single carrier operation. The required base station frequency accuracy for single-carrier in 3GPP is 0.05 ppm for macro base stations, and 0.1 ppm for shorter range base stations. Since the transmitter circuits for different carrier frequencies typically are located on different boards, and no requirement exists on relative frequency error between different carriers, there will most likely be a non-zero relative frequency error between the carriers. UEs may need to compensate for this error in order to achieve good receiver performance on both carriers. U.S. patent application Ser. No. 12/248,756, titled “Relative Frequency Error Compensation for Multi-Carrier Receivers,” filed Oct. 9, 2008, assigned to the assignee of the present application, and incorporated herein by reference in its entirety, addresses this problem in the DL. If multiple carriers are deployed also in the UL, a similar compensation may be needed on the transmitter side in the UE, and/or in the base station, or Node B, receiver.
For legacy, single carrier UEs, the transmitted (UL) carrier frequency is required to follow the received (DL) carrier frequency, with a fixed frequency separation determined by the duplex distance in the operating band. This is accomplished by using the same frequency reference for both receiver and transmitter, and having an automatic frequency control (AFC) algorithm adjusting this common frequency reference to achieve zero frequency error in the received signal. However, if the UE has some non-zero frequency error itself, and/or it is causing some Doppler shift due to movement towards or away from the base station, the Node B receiver may need to compensate for this individually for each UE.
In the case of multiple UL carriers transmitted from the same UE, it is reasonable to assume that the UE should attempt to have its transmitted carrier frequencies follow the corresponding downlink frequencies in a similar manner—per carrier.
There are two basic multi-carrier transmitter architectures to achieve per-carrier frequency following. In the first architecture, separate baseband (BB) and radio frequency (RF) circuits are used for each carrier. In this architecture, the UE operates as parallel legacy UEs, wherein each different carrier signal circuit performs frequency error compensation individually. The deficiency of this solution is the duplication of hardware, which increases cost, complexity, die area and power consumption.
The other architecture is to utilize joint RF and BB processing for all carriers in the same circuit. In this case, signals corresponding to the different carrier frequencies are generated with a fixed, nominal frequency separation. However, state of the art transmitter designs utilizing this architecture do not account for any frequency error between the carriers. Such a solution would increase the perceived frequency error in the Node B receiver, thus potentially degrading demodulation performance.